At present, there are two common problems existing in a high-voltage linear integrated circuit (IC) on the market: 1. a voltage of a Light Emitting Diode (LED) loaded lamp bead is severely restricted by an input voltage, and a total voltage drop of the lamp bead cannot be too much lower than the input voltage. 2. LED illumination stroboflash is severe when the input voltage fluctuates.
In the prior art, a current flowing through an LED lamp string under the control of a control circuit be a constant value in the control of an LED power source, and the constant value does not change along with the change of the input voltage of an external circuit. The control circuit is as shown in FIG. 1. When the input voltage is greater than a set voltage, redundant voltage drop may be applied to the current control circuit IC, so that the temperature of the IC is increased, thereby increasing power consumption, and burning out the IC, when the input voltage is smaller than the set voltage, a charge voltage on a capacitor may be reduced Under the same condition, the discharge time of the capacitor is shortened and the stroboflash of the LED lamp string is severe.
A relationship of a voltage and a current on an output end of a rectifying circuit may be shown in FIG. 2. In FIG. 2, V1 refers to a waveform diagram of a voltage that is rectified and filtered, and I3 refers to a waveform diagram of a current of an LED lamp string. Since the voltage drop on the LED lamp string is a constant value, a voltage waveform V3 of an input end of a current control circuit 50 as shown in FIG. 2 is consistent with the waveform V1.
Generally, a filter circuit 20 may include an electrolytic capacitor C, and thus a magnitude of the voltage V1 is related to the following factors: a magnitude of an input voltage of a power source, a magnitude of an electrolytic capacitor C, and a magnitude of a circuit charge/discharge current.
At a moment t11, a positive voltage V1 of an LED lamp string exceeds a voltage required for conducting the LED lamp string. When the LED lamp string is conducted, the charge current in the filter circuit 20 is the largest, and may be reduced along with the increase of the voltage. A current I3 flowing through the LED lamp string under the control of the current control circuit 50 may be a constant value, and the voltage drop on both ends of the LED lamp string is equal to V11, at a moment t2, the input voltage of the power source reaches a maximum value, the voltage on the filter circuit 20 also reaches a maximum value, the charging of the capacitor in the filter circuit 20 is stopped, and the voltage V3 on the input end of the current control circuit 50 also reaches a maximum value; then, the input voltage is gradually reduced, the capacitor in the filter circuit 20 begins discharging, a sum of a discharge current of the capacitor and a current flowing in from the power source is equal to the current in the LED lamp string to maintain the LED lamp string conducted, and the voltage V3 on the input end of the current control circuit 50 is also reduced accordingly, at a moment t3, the voltage of the power source is lower than the voltage of the filter circuit 20, the power source stops inputting the current, the filter circuit 20 discharges to maintain the LED lamp string conducted, and the voltage of the filter circuit 20 drops; at a moment t31, the voltage V1 of the filter circuit 20 is equal to the conduction voltage V11 of the LED lamp string, and the current in the LED lamp string is reduced along with the decrease of the voltage V1; at a moment t5, the voltage V1 is reduced to a minimum value, the current in the LED lamp string is the smallest, and then the voltage V1 is increased with the increase of the input voltage, and the current in the LED lamp string is also increased; at a moment t51, the voltage V1 reaches the conduction voltage of the LED lamp string again, and the above process is repeated.
At the moments t1 to t3, the voltage V1 changes according to a sine rule, at the moments t3 to t5, the voltage V1 drops according to an exponential curve, and the constant of the discharge time is related to the magnitude of the capacitor, the magnitude of the discharge current, and the like; at the moments t3 to t5, the capacitor in the filter circuit 20 discharges, the magnitude of the discharge current is controlled by the current control circuit 50, and the voltage V1 is reduced.
When the input voltage of the power source is smaller than a nominal voltage, energy stored in the filter circuit 20 may be insufficient to discharge to maintain the input voltage reaching the voltage required for conduction again, which may worsen the stroboflash of the LED lamp string.
When the input voltage is greater than the nominal voltage, the voltages V1 of the filter circuit 20 and a positive end of the LED lamp string are increased. During the charge and discharge of the filter circuit 20, the next charge begins before the previous discharge is finished, so that the voltage on the filter circuit is increased, the voltages V1 are entirely moved up, and are always greater than the conduction voltage of the LED lamp string during a full period, and the current in the LED lamp string is a constant value. The LED lamp string has no stroboflash at this time, as shown by a waveform V12 in FIG. 3. However, a negative voltage V3 of the LED lamp string is also increased, as shown by a waveform V32. That is, since the voltage drop on the current control circuit 50 is increased, the power consumption is increased, and heat is also increased, thereby resulting in sharp increase of temperature and reduction of reliability. In the drawing, V11 refers to a waveform diagram of a positive voltage of the LED lamp string when an input voltage is equal to a nominal voltage, and V31 refers to a waveform diagram of a negative voltage of the LED lamp string.